Thermal compensating structural member

ABSTRACT

A structural member that remains near a constant length when subjected to varying thermal loads. The structure includes an outer bi-metallic housing composed of inner and outer concentrically spaced casings attached to one another at one of their ends and left free to expand and contract at the other of their ends. The casings are made of materials having different coefficients of expansion and therefore their lengths will vary with respect to one another as their thermal environment changes. A compensator strut is slidably mounted in the outer housing. A compensating drive linkage mounted on the free ends of the inner and outer casings so as to be rotated by relative movement between the casings is connected to the strut and causes sliding movement thereof to maintain the structure at a constant length when the components thereof change lengths due to expansion and contraction.

Q United States Patent (151 3,675,376 Belew 1 July 11, 1972 s41 THERMALCOMPENSATING 3,528,206 9/1910 Baird 52/1 STRUCTURAL MEMBER PrimaryExaminer-Henry C. Sutherland [721 n-mrmn'mlk'fla Att0mey-L. D. Wofiord,Jr., c. cv Wells and John R. 731 Assignee: The Unlted sum at America asManning represented by the Administrator of the National Aeronautics andSpace Adminis- [57] ABSTRACT A structural member that remains near aconstant length when [22] Filed: Feb. 12, 197i subjected to varyingthermal loads. The structure includes an Appl. No.: 114,847

outer bi-metallic housing composed of inner and outer concentricallyspaced casings attached to one another at one of their ends and leftfree to expand and contract at the other of their ends. The casings aremade of materials having different coefficients of expansion andtherefore their lengths will vary with respect to one another as theirthermal environment changes. A compensator strut is slidably mounted inthe outer housing. A compensating drive linkage mounted on the free endsof the inner and outer casings so as to be rotated by relative movementbetween the casings is connected to the strut and causes slidingmovement thereof to maintain the structure at a constant length when thecomponents thereof change lengths due to expansion and contraction.

7 Claims, 6 Drawing Figures PHENTEDJUL 1 1 I972 SHEET 10F 3 FlG. l

ROBERT R. BELEW INVENTOR BY M ATTORNEY PATENTEDJUL I 1 m2 SHEH30E3INVENTOR W E L E B R T R E B O R ATTORNEY THERMAL COMPENSATINGSTRUCTURAL MEMBER ORIGIN OF THE INVENTION The invention described hereinwas made by an employee of the United States Government and may bemanufactured and used by or for the Government for governmental purposewithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION l Field of the Invention The inventionrelates generally to structural members designed to maintain a constantlength when subjected to a changing thermal environment. Moreparticularly the invention relates to such members employing bimetalliccomponents having different coefficients of expansion in conjunctionwith a compensating drive linkage to achieve constant length duringthermal changes.

2. Discussion of the Prior Art There are numerous applications forstructures in which dimension changes due to expansion and contractioncan be kept to a minimum; one important application being forfabrication of various types of antennas designed for use in orbit aboutthe earth. Such an antenna is subjected to greatly varying thermalloads, depending upon whether it is exposed or shaded from the sunsradiation, and the antenna will be distorted as the structuralcomponents thereof expand and contract. Distortion affects the antennashape which is critical to transmitting efficiency thereof so distortionis an important factor to be considered when designing an antenna,particularly one for use in space where thermal variations are extreme.

Heretofore the approach employed to achieve acceptable, or at leastoptimum, dimensional stability for antennas has been to use surfacecoatings to control the thennal emissivity and reflectivity of thestructure surfaces and thus, to some extent, the expansion andcontraction of the antenna. The disadvantage of this solution is thatthe effectiveness of surface coatings is dependent upon a predictableset of conditions, such as structural surface orientation with respectto the sun or other source of thermal energy and precise information asto possible changes in the thermal environment.

SUMMARY OF THE INVENTION It is an object of the invention to provide astructural member that remains a substantially constant length duringchanges in the thermal environment that result in expansion andcontraction of the materials from which the member is made.

Another object of the invention is to provide a structural member havingbi'metallic tubular components and employing the lever principle toachieve near constant length under a varying thermal environment.

The invention that accomplishes the above and other objects is a tubularstructure that can be used as a cross-member in an antenna structure.The invention includes an outer housing comprised of an inner and outercasing; the two casings being joined at one end and left free at theirother end to expand and contract. The casings are made of dissimilarmaterials having different coefficients of expansion and therefore theirlengths will vary with respect to one another as their thermalenvironment changes. A compensator strut is slidably mounted in theouter housing and this strut is connected to and moved relative to theouter housing by a compensating drive linkage. The linkage includes alever that is pivotally attached to both the inner and outer casing andto the compensator strut whereby relative movement between the twocasings will pivot the lever and slide the compensator strut in or outof the outer housing an amount that compensates for any expansion orcontraction of the outer housing, thus keeping the overall length of thestructure constant. By choosing appropriate materials for the outerhousing and the compensator strut, and a suitable lever length andmounting, the compensator strut can be moved relative to the outerhousing just enough to compensate for length changes in the structureand maintain the overall structure at a constant length.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a pictorial view of aspacecraft with a deployed antenna of a type which could utilize crossmembers and the like constructed in accordance with this invention.

FIG. 2 is a pictorial view of a neanconstant-length tubular structure.

FIG. 3 is a cross-sectional view of the structure illustratedpictorially in FIG. 2.

FIG. 4 is a cross-sectional view taken along lines 4-4 of FIG. 3.

FIG. 5 is a cross-sectional view of a structure similar to that shown inFIG. 3 except that the inner casing is adapted for connection to asupporting structure rather than the outer casing as in FIG. 3.

FIG. 6 is a cross-sectional view of an alternate embodiment of theinvention wherein three concentrically arranged tubular members areemployed to achieve length stability.

DETAILED DESCRIPTION OF THE INVENTION A space station I0 is shown havingan antenna structure l2 mounted thereon that includes a dish-shapedframework I4 that is composed of numerous cross members. The dishshapedframework is constructed to support a reflecting mesh (not shown) in aparticular curvature, parabolic being a commonly used curvature, andthis curvature is critical to peak transmitting efficiency. It can bereadily understood how thermal changes that cause the cross'membersmaking up the antenna to expand and contract would result in dimensionalchanges that would affect the curvature of the antenna. It is alsoobvious that the antenna design would be greatly facilitated bycross-members that maintain a near constant length when subjected tochanges in thermal environment.

In FIG. 2 a thermal compensating structural member 15 is shown having aconnecting eye 16 on one end thereof that can be connected to anassociated structure and a mounting flange 17 on the other end also forattachment to an associated structure. The connecting eye and flange arejust one type of mounting that was chosen for illustration and it shouldbe understood that the member could be provided with any type ofmounting bracket or the like deemed desirable or necessary.

Structural member 15, shown more clearly in cross-section in FIG. 3,includes an elongated hollow body composed of outer casing 18 and aninner casing 20. End 22 of casing 20 is joined to end 24 of casing 18.Ends 26 and 28 of casings l8 and 20 are left free to expand and contractrelative to one another when the structural member is subjected tothermal changes. Casing I8 is shown perforated to expose casing 20 tosolar radiation. Casing 20 is not perforated, however perforations couldbe employed if warranted. To assure uniform heat conduction to bothcasings high conductivity heat paths can be provided between theadjacent casing walls by bonding foil strips 21 (see FIG. 5) of aluminumor other suitable heat conductive material between the casings. Foilstrips could likewise be employed to provide high conductivity heatpaths between members 20 and 36. The foil strips are of sufficientlength to allow for expansion or contraction of the casings withoutrupturing of the foil.

One end of the hollow body is substantially closed by a plug 30 having acollar 32 mounted in the center thereof. Collar 32 has a sleeve bearing34 mounted therein in which end connector l6 slides. End connector 16 isbolted to one end of a tubu lar compensator strut 36 supported in thecenter of the hollow body. A drive linkage 35 is connected to the otherend of strut 36 by means of a bracket 38 attached to strut 36. Bracket38 has pins 40 and 42 mounted thereon. The drive linkage includes alever 44 with one of its ends, end 46, mounted on a pin 40 and the otherend, end 48, attached to casings l8 and 20. Casing 18 has a hinge $0suitably mounted thereon (see FIG. 4) by brazing or the like and casing20 has a similar hinge 52 mounted thereon. Pins 54 and 56 supported inhinges 50 and 52 are mounted through holes formed in end 48 of lever 44for pivotally mounting the lever to casings l8 and 20.

The link illustrated in FIG, 3 would function as follows, assuming thatcasing 20 is composed of aluminum, casing 18 of titanium, and thethermal environment is changing from cold to warm so that the variouscomponents of the structural member will expand. The thermal coefficientof expansion of the aluminum casing is about 2.75 times that of thetitanium casing so that upon expansion there will be more elongation ofcasing 20 than of casing 18. This will cause pin 56 to move to the right(as viewed in FIG. 3) relative to pin 54 and will result incounterclockwise rotation of link 44. This counterclockwise movementwill move strut 36 to the right and reduce the overall length of thestructural members. By proper selection of materials for casings l8 and20, and strut 36, and by choosing a suitable lever length and spacing ofthe pins 54, 56 and 40, strut 36 can be retracted an amount that willcompensate for expansion of the member and maintain a near constantlength during changes in the thermal environment.

The link shown in FIG. is essentially the same as that shown in FIG. 3and functions similarly. in FIG. 5, inner casing has been provided witha mounting flange rather than outer casing 18. Also, strips ofconducting foil 21 have been connected between casings l8 and 20 tofacilitate heat transfer therebetween. In addition, strut 36 is biasedto the right, as viewed in FIG. 5, by means of a helper spring 58 thatbears against pin 42. The spring could be attached in any suitablemanner to an associated structure (not shown) so as to be placed intension. The helper spring would be used to decrease the load on thedrive linkage and would be employed with a structural member loaded intension. The structural member shown in FIG. 3 could be provided with ahelper spring if desired.

An alternate embodiment of the invention is illustrated in FIG. 6wherein structural member 60 includes an outer perforated casing 62, anintermediate casing 64 and an inner casing 66. A plug 68 attached to oneend of casing 66 closes one end of the member and a second plug 70attached to one end of casing 62 closes the other end of the member.Plugs 68 and 70 include mounting collars for attaching member 60 toassociated structures, but other types of mounting means could beemployed if desired. In this type of member casings 62 and 66 are of thesame material, a material of low thermal expan sion coefficient such astitanium; and casing 64 is made of a material of substantially higherthermal expansion coefficient such as aluminum. By proper selection ofcasing lengths the expansion and contraction of the casings can be usedto offset one another and maintain the overall length of structure 60nearly constant.

What is claimed is:

l, A near constant length thermal expansion structure comprising:

an elongated hollow body having one end thereof adapted to be attachedto an associated structure and supporting means mounted on the other endthereof;

an elongated compensator strut slidably mounted in said hollow body,said compensator strut including an end connector mounted on end thereofthat is slidably supported in said supporting means, said end connectorbeing adapted for connection to an associated structure;

said elongated hollow body including an outer casing and an inner casingmounted in said outer casing in spaced and concentric relation withrespect thereto, said inner and outer casings being composed ofmaterials having different coefficients of expansion so that thermalchanges will result in the inner and outer casings achieving dif ferentlengths due to contraction and expansion; and

linkage means mounted on said hollow body and actuated by relativemovement between said inner and outer casings, said linkage means beingconnected to the other end of said compensator strut op site theslidably supported end to move said strut rela ve to said hollow body anamount determined by the relative amounts of movement between said innerand outer casings.

2. The structure recited in claim 1 wherein said inner and outer casingsare attached to one another at one of their ends so as to remain fixedrelative to one another and the other ends of said casings are left freeto expand and contract to different lengths when subjected to thermalchanges, the free ends of said casings being connected to said linkagemeans for imparting movement thereto.

3. The structure recited in claim 2 wherein said linkage meanscomprises:

a first hinge mounted on the free end of said outer casing,

a second hinge mounted on the free end of said inner casing in radialalignment with respect to said first hinge and spaced therefrom; and

a lever having one end thereof pivotally mounted in said first hinge,the other end pivotally mounted to said compensator strut andintermediate the ends thereof said lever being pivotally mounted to saidsecond hinge; whereby, relative movement of said inner and outer casingswill cause said lever to pivot and thereby move said compensator strutrelative to said hollow body.

4, The structure recited in claim 3 wherein said inner casing isaluminum and said outer casing is titanium.

5. The structure recited in claim 4 wherein said compensator strut is anelongated tubular body and further comprises bracket means mounted onsaid other end of said compensa tor strut for pivotally mounting saidother end of said lever to the compensator strut; whereby, pivotalmovement of said link means will slide said compensator strut relativeto said inner and outer casing.

6. The structure recited in claim 5 which further includes heatconducting means connected between said inner and outer casing tofacilitate heat transfer therebetween.

7. The structure recited in claim 6 wherein said heat conducting meansare foil strips fastened to said inner and outer casing.

1. A near constant length thermal expansion structure comprising: anelongated hollow body having one end thereof adapted to be attached toan associated structure and supporting means mounted on the other endthereof; an elongated compensator strut slidably mounted in said hollowbody, said compensator strut including an end connector mounted on endthereof that is slidably supported in said supporting means, said endconnector being adapted for connection to an associated structure; saidelongated hollow body including an outer casing and an inner casingmounted in said outer casing in spaced and concentric relation withrespect thereto, said inner and outer casings being composed ofmaterials having different coefficients of expansion so that thermalchanges will result in the inner and outer casings achieving differentlengths due to contraction and expansion; and linkage means mounted onsaid hollow body and actuated by relative movement between said innerand outer casings, said linkage means being connected to the other endof said compensator strut opposite the slidably supported end to movesaid strut relative to said hollow body an amount determined by therelative amounts of movement between said inner and outer casings. 2.The structure recited in claim 1 wherein said inner and outer casingsare attached to one another at one of their ends so as to remain fixedrelative to one another and the other ends of said casings are left freeto expand and contract to different lengths when subjected to thermalchanges, the free ends of said casings being connected to said linkagemeans for imparting movement thereto.
 3. The structure recited in claim2 wherein said linkage means comprises: a first hinge mounted on thefree end of said outer casing, a second hinge mounted on the free end ofsaid inner casing in radial alignment with respect to said first hingeand spaced therefrom; and a lever having one end thereof pivotallymounted in said first hinge, the other end pivotally mounted to saidcompensator strut and intermediate the ends thereof said lever beingpivotally mounted to said second hinge; whereby, relative movement ofsaid inner and outer casings will cause said lever to pivot and therebymove said compensator strut relative to said hollow body.
 4. Thestructure recited in claim 3 wherein said inner casing is aluminum andsaid outer casing is titanium.
 5. The structure recited in claim 4wherein said compensator strut is an elongated tubular body and furthercomprises bracket means mounted on said other end of said compensatorstrut for pivotally mounting said other end of said lever to thecompensator strut; whereby, pivotal movement of said link means willslide said compensator strut relative to said inner and outer casing. 6.The structure recited in claim 5 which further includes heat conductingmeans connected between said inner and outer casing to facilitate heattransfer therebetween.
 7. The structure recited in claim 6 wherein saidheat conducting means are foil strips fastened to said inner and outercasing.